The Zika virus that has thus far affected close to 50 countries will likely continue to spread and pose a “formidable” challenge to eradicate, the head of the World Health Organization (WHO) said Wednesday.

The mosquito-borne disease presents a “bigger menace” than any other recent epidemic and will possibly “get worse before it gets better,” Margaret Chan told Agence France-Presse during a fact-finding mission in Brazil.

Other major disease outbreaks of recent years like Ebola and severe acute respiratory syndrome (SARS) may have been deadlier than Zika, whose symptoms resemble the flu. However, Chan said, they only affected nine and 26 countries respectively, whereas Zika has already been reported in 46 countries. Moreover, the Aedes aegypti mosquito that passes on the virus exists in at least 130 countries, significantly increasing the possibility of a further spread.

Zika has also been linked with the birth defect microcephaly in children born to women with the disease, as well as a paralysis disorder called Guillain-Barre syndrome, but neither relationship has been conclusively proved and much about the disease remains unknown.

“We are dealing with a tricky virus, full of uncertainties, so we should be prepared for surprises,” Chan said.